1. Field of the Invention
This invention relates to antilock brake systems; and more particularly, to antilock brake systems utilizing brake system pressure feedback to control brake force.
2. Description of the Related Art
Antilock brake systems typically comprise a feedback control that modulates brake system pressure in response to monitored wheel velocity to control the percentage of wheel slip during braking. The objective of the antilock system is to maximize utilization of available tire traction thereby minimizing stopping distance while maintaining vehicle stability. Elements of a typical antilock system include; a treadle valve (operated by the drivers foot), a system pressure modulation device, a wheel speed sensor and an electronic control unit (ECU). During braking a wheel lock condition is sensed by the ECU when the vehicle wheel speed achieves a predefined value of deceleration. When such a condition is detected, the system pressure modulation device reduces system pressure permitting the wheel to roll back up from the locked condition. At the beginning of the braking operation, the ECU produces a reference velocity which is a projection based upon the braking performance the vehicle is capable of achieving. Thus, the reference velocity is an artificial signal which is a projection of vehicle speed during a braking operation. When the wheel rolls up to the reference velocity, the system pressure modulation device increases system pressure thereby reapplying braking force. This process is repeated until the vehicle is stopped or the operator releases force on the treadle valve. Problems associated with known antilock systems include;
1. In a braking condition in which excess brake force is available (i.e., the vehicle is lightly loaded or the coefficient of friction on the road surface is low) the brake system pressure rise rate may easily exceed the brake system pressure necessary to appropriately control the vehicle wheel velocity. Therefore, the vehicle wheels are easily locked. Hysteresis in the braking system results in a delay in the apply and the release of brake force, and therefore, extends the time that the vehicle wheel is excessively braked. This increases stopping distance and reduces vehicle stability. PA1 Therefore, there is a need for a system that modulates the rise in system pressure to a rise rate which matches the response of the system to avoid or minimize brake pressure overshoot thus avoiding excessive brake force overshoot. PA1 2. When antilock systems reapply after a first cycle, the brake pressure rise rate again may easily exceed the brake system pressure necessary to appropriately control the vehicle wheel velocity. PA1 Therefore, there is a need for an antilock system which stores the system pressure at which excess brake force occurred and uses this value in subsequent braking cycles to creep up to an appropriate brake force condition. PA1 3. When antilock systems reduce system pressure in response to detection of an excess brake force condition, the system pressure is typically reduced to a target of zero psig. PA1 When the wheel velocity minimum is reached and the ECU generates a command to raise system pressure to reapply brake force, the brake system pressure is often well below the brake system pressure which would generate the appropriate brake force for the respective condition. Time is lost in raising system pressure to an appropriate value. PA1 Therefore, there is a need for an antilock system which will reduce brake system pressure to a value which minimizes the time lost to rebuild pressure in response to an ECU command to reapply brake force.
1) as fast as possible; or, PA2 2) according to a fixed schedule.